This invention relates to electrochemical gas sensors and, more particularly, to an electrochemical gas sensor which is substantially more accurate than prior art sensors.
A wide variety of electrochemical gas sensors have been designed over the years to measure the concentration, or partial pressure, of a particular gas in a mixture of gases. One application for such a sensor is to provide an indication of the concentration of oxygen in gaseous mixtures.
One class of such sensors includes a sensing electrode, a counter-electrode, an electrolyte in contact with both electrodes, a membrane adjacent a surface of the sensing electrode, and a path for the gas to be measured to dissolve into and diffuse through the electrolyte. When the dissolved gas diffuses through the electrolyte and contacts the sensing electrode, a measurable current can be detected in an external circuit connected between the electrodes. In a galvanic type sensor, the measurable current flows without application of an external voltage, while a polarographic type sensor requires the application of an external voltage in order to produce the measurable current. The present invention is applicable to both types of sensors. Examples of sensors of the type described above are disclosed in U.S. Pat. Nos. 3,429,796, issued Feb. 25, 1969; and 3,767,552, issued Oct. 23, 1973, both assigned to the assignee of the present invention.
An important consideration in the design of a gas sensor is that the output signal derived from the cell be proportional to the partial pressure of the measured gas in the gas mixture. However, in most of the prior art sensors, the output signal is not only responsive to the incoming gas concentration, but is also responsive to the gas previously dissolved in the bulk of the electrolyte distributed between the electrodes. This dissolved gas is generally the result of previous exposure of the sensor to a high concentration of the gas. Because the output signal provided by prior art sensors includes a component related to previously dissolved gas in the electrolyte, the accuracy of these sensors is a function of the gas exposure history of the sensor. This accuracy limitation severely limits the application of these prior art sensors in the detection of low concentrations of gas.
Several attempts have been made to eliminate the sensor errors caused by gas dissolved in the bulk of the electrolyte. One method employs the addition of a scavenger electrode positioned in the bulk of the electrolyte. The scavenger electrode is connected to an electrical circuit so that it acts in essence as a second sensing electrode.
The object of the scavenger electrode is to reduce (or oxidize) the gas in contact with it before that gas diffuses through the bulk electrolyte to the sensing electrode, thus eliminating sensing error. In order for the scavenger electrode to be effective, it must prevent any of the dissolved gas from diffusing back to the sensing electrode. Unfortunately, the electrochemical reaction which consumes the dissolved gas can only occur at the surface of the scavenger electrode. Thus, the scavenger does not have an effect on gas which diffuses through or around it. Accordingly, the scavenger electrode mechanizations do not eliminate the sensing errors to the degree necessary for accurately measuring extremely low concentrations of gas. An example of the scavenger electrode mechanization described above is disclosed in U.S. Pat. No. 3,454,485, issued July 8, 1969 to P. Hauk, et al.
Accordingly, it is an object of the present invention to provide a new and improved electrochemical gas analyzer.
It is another object of the present invention to provide an electrochemical gas analyzer capable of providing an output signal which is not responsive to gas previously dissolved in the analyzer electrolyte.
It is yet another object of the invention to provide a gas analyzer having a compensation electrode which produces a signal related to the gas previously dissolved in the electrolyte.